Copyright Statement

Abstract

Offshore operations often require heavy subsea equipment,
such as suction piles or cans, to be lowered by a support vessel
into the sea. A lifting device must have adequate capacity to
withstand the dynamic loads generated by the motions of the
vessel and the heave response of the structure.
The objective of this study is to determine the added mass
and damping of a suction can oscillating in beave near the free
surface; knowledge of these hydrodynamic properties is
required for the accurate prediction of the dynamic lift forces
during the deployment. This project is a logical progression
following two similar studies, which investigated these
hydrodynamic properties for the suction can in the mid-water
position and when approaching the seabed. All three studies
involved the conduct of model tests to determine the
hydrodynamic properties. Free decay tests were conducted at
several heave frequencies, and the added mass, linear and
quadratic damping components were determined. In addition,
the effect of varying the percentage of open hatch area has been
investigated.
rest data demonstrates that the heave added mass is
strongly dependent on the frequency of motion, and its values
are significantly smaller thanthose measured in the unrestricted
flow. From observations, there was no dependency on the
motion amplitude, nor did the size of open hatcbes have notable
effect on the added mass. It was observed that wben the top
plate of the structure was in contact with the free surface a
mean "pull down" force appeared. This force is caused by the
suction underneath the top plate when the can moves upwards. As opposed to the mid-water position and near the sea floor, the
study indicates that the area of open hatches has no noticeable
effect on the heave damping when the suction can is oscillating
near the free surface.